MTSU reduces concussion risk with padded helmet covers
Football Helmet Shell Add-On Testing
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries
NFL exec suggests Guardian Caps worn to protect players during practices could one day debut in games
CFL players adjust to protective helmet caps
What are Guardian Caps? Shadow Hills football using new device to reduce head injuries
NFL Mandates Guardian Caps During Training Camp - Team Insight
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Guardian Cap - Soft-Shell Protective Helmet Cover – Mario Luna Football
Guardian Caps: Why some NFL players are wearing them in training camps